Mask inspection, or critical optical component (e.g., reticle, wafer or the like) inspection, is an operation of checking the correctness of the fabricated masks or critical optical components. Certain inspection systems may utilize wavelengths 350 nm or below, or broadband deep ultraviolet (DUV) in the range of 180-450 nm wavelength, or wavelengths either narrow or broadband below 200 nm down to 1 nm (e.g., including EUV (13.5 nm), e-beam systems or the like).
The high-energy photons utilized by such inspection systems may interact with contaminating compounds or by the photon-induced dissociation of adsorbed hydrocarbons on the critical surfaces. Photon-induced contamination on the surfaces may arise from organic, inorganic compounds (including acids and bases) or metal compounds. This may result in undesirable growth of thin contamination films on optical elements or critical surfaces or critical components being inspected. That is, the critical surfaces being inspected may be contaminated during the inspection process.
In the absence of radiation, mask or wafer can become contaminated. Contamination may come from packaging, storage, ambient, handling, transport and from the loading process. The buildup of contamination on a mask, wafer or an optical element substrate is typically on the order of a few monolayers. Contamination growth and contamination rates depend largely on the initial amount of contaminates that exist on the mask or optical element that will be inspected. With some contamination present on the surface, photon-induced contamination easily follows when optical element is exposed to ionizing radiation. Experimental data indicates that a very modest increase of the mirror temperature can dramatically decrease the equilibrium concentration of an adsorbate and, thus, decrease the rate of carbon growth. In many cases, films from photon-induce processes cannot be easily removed and may require specific wavelength and radiation to remove. In addition, photon-induced damage of the mask or wafer may occur after the contamination deposition of a few atomic layers.
Therein lies a need for a method and apparatus for preventing or minimizing contamination on a critical surface.